This invention relates to chlor-alkali cell diaphragms and to procedures for treating such diaphragms with magnesium compounds to improve their operating characteristics.
In the electrolysis of aqueous sodium chloride solutions or other brines to produce chlorine and caustic, one of the principal types of equipment used has a porous asbestos diaphragm separating the anode and cathode chambers. The anode can be provided as graphite, a dimensionally stable or adjustable metal anode or as other types known in the art. The cathode is typically a woven wire mesh screen. The diaphragm can be formed directly on the side of the cathode facing the anode chamber by vacuum deposition of asbestos and binders by techniques similar to those used in paper making. The deposited diaphragm is normally heated to fuse the binder.
The diaphragm must be porous enough to permit the flow of brine from the anode chamber into the cathode chamber under a small hydrostatic head of pressure. But it should also inhibit the diffusion of hydroxyl ions from the cathode chamber back into the anode chamber. The flow of the brine from the anode chamber to the cathode chamber aids in minimizing diffusion from the cathode chamber back into the anode chamber. Also, excessive leakage of hydrogen or chlorine gases through the diaphragm could contaminate the products being produced and require costly purification or even produce hazardous mixtures of the two gases. Although the nature of asbestos is not completely understood, it has been theorized that hydroxyl ion diffusion is inhibited by negative charges and a concentration of hydroxyl ions in the hydrated magnesium silicate at the surface of the asbestos. These features, combined with the chemical resistance of asbestos, make it a desirable component of chlor-alkali cell diaphragms.
However, chlor-alkali cell diaphragms made only or mainly of asbestos have a short life. The cathode chamber has a highly basic pH, such as 11-14, while the anode chamber has an acid pH, such as 3-5. Combined with the flow of brine through the diaphragm, these factors cause erosion and chemical and dimensional changes in an asbestos diaphragm, requiring replacement of the diaphragm when the cell becomes too inefficient.
Asbestos diaphragms have been improved by using various binders and modifiers. Fluorocarbon resins such as polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene and hexafluoropropylene, known as fluorinated ethylene-propylene (FEP), are effective as binders, due in part to their chemical inertness. Such polymers can be provided as an aqueous codispersion with asbestos from which the diaphragm is deposited. See U.S. Pat. No. 3,928,166--0'Leary et al. (1975) and U.S. Pat. No. 4,070,257--Motani et al. (1978). Fibers of such resins can also be used in the dispersions. Upon heating to fuse the fluorocarbon resin, the binder adheres to the asbestos in places, generally without completely coating the asbestos. Leaving much of the surface of the asbestos exposed is desirable since asbestos is hydrophilic, that is it wets readily, aiding the brine in flowing through the diaphragm, and it is thought that its surface characteristics can inhibit the back diffusion of hydroxyl ions.
In addition to such fluorocarbon resins which are hydrophobic, fluoropolymer resins containing hydrophilic functional groups such as carboxylic, sulfonic and phosphonic groups can be used as asbestos diaphragm modifiers. They can completely coat the asbestos, substituting their own functional groups for the surface charge and hydrophilic characteristics of the asbestos which then functions as a stable filler. Such resins can react with the asbestos rather than merely sticking to it, as discussed in Dutch Pat. No. 69/17096 (1970) and British Pat. No. 1,286,859 (1972), both to Grot, and U.S. Pat. No. 3,853,721--Darlington et al. (1974).
Each of the developments of the prior art is less than ideal. The fluoropolymer resins with functional groups are generally more expensive than fluorocarbon resins without the functional groups. Diaphragms with exposed asbestos remain subject to attack. Also, magnesium compound tends to be dissolved from the asbestos fibers themselves at the acid (anode) side of the diaphragm and be deposited as magnesium hydroxide on the basic (cathode) side of the diaphragm. This causes restrictions in the size of pores through the diaphragm and sooner or later can clog the pores to the point where the diaphragm is no longer useful. Alternatively, fine particle size magnesium hydroxide can be washed all the way through the diaphragm, leaving a silicate surface. Excessive flow rates and voltages can result. See "Fundamentals of Diaphragm Performance" by van der Heiden, pp. 33-40 of "Diaphragm Cells for Chlorine Productions--Proceedings of a Symposium Held at University City, London, England, June 16 and 17, 1976," published by the London Society of Chemical Industry, 1977.
U.S. Pat. No. 4,007,059--Witherspoon et al. provides a fuel cell diaphragm comprising asbestos, PTFE, FEP and alkaline earth metal oxide. However, such a fuel cell has a strongly basic environment throughout, and there is no flow of brine through the diaphragm as in a chlor-alkali cell.